I did return it, yesterday. To Apple's credit, they did not give me a hassle, but there was no attempt to understand the issue or offer a path forward. Originally I was going to just wait 3 months and try again. Problem is, I did get to experience what 18-cores could do for my workflow. Grin.Return it and get the same config if you wanna stay on macOS.
So, I gave the entire experience some thought overnight. After exclusively buying Apple hardware for 15 years, I decided it was time to give PCs a try. I ordered a similarly configured PC tower this morning. From a productivity perspective, there's nothing I'm using that is MacOS exclusive. Even if the PC gives me trouble, at least I can swap out hardware pieces and move forward. In the case of the iMac Pro, I either hoped my render would finish or... well, no, I only had one option.
I'm not a fan of Windows, but I need to keep making a living. I can't let a chip that adds almost no value to an $11,500 machine be the reason I miss deadlines.